RTC: Fix the cross interrupt issue on rtc-test.

The rtc-test driver is meant to provide a test/debug code for the RTC
subsystem.

The rtc-test driver simulates specific interrupts by echoing to the
sys interface. Those were the update, alarm and periodic interrupts.

As a side effect of the new implementation, any interrupt generated in
the rtc-test driver would trigger the same code path in the generic
code, and thus the distinction among interrupts gets lost.

This patch preserves the previous behaviour of the rtc-test driver,
where e.g. an update interrupt would not trigger an alarm or periodic
interrupt, and vice-versa. In real world RTC drivers, this is not an
issue, but in the rtc-test driver it may be interesting to distinguish
these interrupts for testing purposes.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Alessandro Zummo <a.zummo@towertech.it>
CC: Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br>
CC: rtc-linux@googlegroups.com
Signed-off-by: Marcelo Roberto Jimenez <mroberto@cpti.cetuc.puc-rio.br>
Signed-off-by: John Stultz <john.stultz@linaro.org>